This invention relates to a method and apparatus for measuring blood flow using an indicator such as a saline solution, and, more particularly, relates to such a method and apparatus using an improved conductivity cell positioned at the tip of a catheter.
As shown, for example, in U.S. Pat. No. 4,572,206 to Geddes et al., a tetrapolar conductivity cell mounted on the tip of a catheter can be used to determine cardiac output or volumetric blood flow. It has been found through the course of work in this area that the construction of the catheter conductivity cell plays a role in the accuracy and repeatability of such determinations. Cardiac output measurements taken with an individual catheter conductivity cell of the type described in the aforementioned Geddes et al. patent were found to vary over time in a controlled environment, and subsequent efforts to improve the repeatability resulted in identification of certain structural features as contributing to performance problems. Stray and distributed capacitance between the conductors along the length of the catheter, and positioning of the conductivity cell leads within the body of the catheter, were identified as sources of repeatability errors associated with the measurement of cardiac output using the dilute saline indicator technique. Additionally, the surface area of the conductivity cell electrodes in contact with the blood appeared to have some effect on performance and repeatability of the catheter conductivity cell.
Newbower and Troutman described a different type of conductivity cell for measuring cardiac output in "Sensor for Catheter-Based Measurement of Electrical Conductivity", IEEE Transactions Biomedical Engineering (1986), BME 33:182-188. The Newbower/Troutman device was patented in U.S. Pat. No. 4,380,237. Newbower and Troutman took a fundamentally different approach to that suggested in the aforementioned patent to Geddes et al., in that their conductivity cell is side-looking, i.e., its electrodes are arranged on the side of the catheter body rather than across the tip of the catheter. The Newbower/Troutman location for the conductivity cell runs the risk of producing large errors in resistivity measurement when the catheter comes close to the wall of a blood vessel, as frequently occurs. Moreover, the Newbower/Troutman design fails to deal with the electrical shunting effect due to the capacitance present between the four electrode wires within the catheter body. It is now believed that capacitance attributable to the four electrode wires within the catheter results in deviations in the conductivity cell constant from what would be theoretically calculated according to the geometry of the electrodes such that the Newbower/Troutman device would, like the aforementioned Geddes et al. device, exhibit less than optimal repeatability.